Molecular Dynamics Studies Of Deformation Mechanism Of Crystalline Metals And Metallic Glasses At High Strain Rate

For material science and mechanics research,it has always been the research focus that deformation mechanism of metallic materials under extreme conditions of high temperature,high pressure,high strain rate and large strain.The focus and difficulty of deformation mechanism study of metallic materials include quantitative analysis of plastic deformation from the microscopic level,establishment of dynamic evolution model of defects,determining the relationship between microstructure and deformation mechanism.This dissertation takes single crystal copper and iron,nanocrystalline copper and Cu-Zr metallic glass as the research object.Using molecular dynamics simulations,the mechanism of microscopic deformation under uniaxial tension and shock compression at high strain rate are studied.Fristly,the effect of different grain sizes on deformation mechanism and temperature change of nanocrystalline Cu under uniaxial tension and shock compression are studied.It is found that the change of mechanical properties and deformation mechanism of nanocrystalline Cu occur in the grain size of about 15nm.The dislocation motion dominates the deformation process when the grain sizes of nanocrystalline Cu are larger than 15nm.As the grain sizes decrease below 15nm,the grain-boundary sliding and rotation become a dominant deformation mechanism.This change of deformation mechanism is the fundamental reason for softening,which is so-called reverse Hall-Petch relationship.According to previous study and molecular dynamics simulation results,combining the grain coalition and the grain-boundary rotation,an ideal deformation mechanism model is established at small grain sizes.The temperature change decreases with the decrease of grain size,and the temperature change caused by grain boundary movement is smaller than that of dislocation movement.It is found that the grain boundary can move almost unimpeded when the temperature is higher than 600K.Explain the phenomenon that metal jet show mobility with relatively low temperature.Secondly,a quantitative calculation method of dislocation velocity in single crystal Cu under shock compression is proposed.It is found that supersonic dislocations can not form in single crystal Cu,and the longitudinal wave velocity of Cu is the upper limit of dislocation velocity.The results show that under increasing shock pressure,we can detect three distinct phases of the defect regimes for single crystal Cu:dislocations(<30 GPa)?stacking faults(30 GPa-49 GPa)?twins(>61 GPa).When the shock pressure is less than 30 GPa,the motion of dislocations and dislocation loops are significant mechanism in single crystal Cu under the shock compression.A new mechanism of nucleation and development of the dislocation loop is proposed.When the shock pressure is greater than 30 GPa,large number of intersecting stacking faults are formed.Dislocations can not slip over long distances due to stacking faults.At a relatively high pressure(>61 GPa),the stacking faults and twins exist together in single crystal Cu.It is shown that two types of stacking faults with vertical directions are the competitive mechanism.When one of them dominates,twins will be formed from this type of stacking fault.Thus,the directions of twins are randomly distributed in the initial stage.Rotation and combination of twins are significant mechanisms,and these mechanisms lead to the final formation of twins.Thirdly,molecular dynamics simulations have been performed on solidification processes of liquid metal Fe by using crystal-liquid configuration method at two different cooling rates.In this way,it simulates the process that metal Fe is treated by laser beam.It is found that the lattice structure of the metal Fe atoms after laser beam treatment in the final state is consistent with that of the untreated atomic structure,which both are bcc crystals.In the final state,the metal Fe under laser beam treatment contains a small amount of high-energy atomic clusters,which make the energy of the local region relatively high.These high-energy atomic clusters affect atomic motions and have different mechanical properties relative to the original metal Fe.The Fe-C alloy system model is established at atomic level.During the process of uniaxial tension,the structure heterogeneity of Fe-C alloy system is induced by atom C.There is relatively larger stress in local area.Dislocation firstly appears at the influenced area of atom C and it induces the phase transition from bcc to fcc/hcp.The change of structure results in the decrease of system strength.The influenced area of atom C is relatively tender in Fe-C alloy system,combining the influence of the phase transition,so it is easier that the failure fracture occurs.Finally,the Cu-Zr metallic glass models with different Cu content are established by rapidly cooling the Cu-Zr metallic alloy liquid.The shock response of CuxZr100-x(x=30,50 and 70)metallic glasses is characterized using large-scale molecular dynamics simulations.Independent of composition,the metallic glasses exhibit the following shock wave propagation regimes:(1)single elastic shock wave for Up<0.25km/s,(2)split elastic and plastic shock waves for 0.25<UP<0.75km/s and(3)overdriven plastic shock wave with a narrow elastic precursor for Up>0.75km/s.Within the split wave and overdriven regimes,the amplitude of the elastic precursor increases with increasing shock intensity,thereby indicating a pressure-dependent yield criterion.Hugoniot states are strongly dependent on the Cu content of the MG with Cu70Zr30 exhibiting a much higher resistance to plastic deformation than either CusoZrso or Cu30Zr70.<0,0,12,0>Voronoi polyhedron has the largest shear resistance,and its content can basically reflect the structural change characteristics of plastic deformation.<0,2,8,1>and<0,3,6,3>have the least shear resistance.Shear transformation zone nucleation and development are the fundamental deformation mechanism for Cu-Zr MG under shock composition.The deformation processes of metal crystal and metallic glass under uniaxial tension and shock compression are studied in this dissertation.It helps to deeply understand the plastic deformation mechanism of metal materials.And it has reference significance for revealing the relationship between microstructure and mechanical properties of metallic materials.